The pathogenesis of herpesvirus infections in man and in experimental animals is investigated by characterization of viral isolates and examination of humoral and cell-mediated immune responses. Rapid, convenient assays have been developed for the detection of Epstein-Barr virus in vitro, which are suitable for clinical use. Patients with infectious mononucleosis lack the ability to suppress outgrowth of EBV infected lymphocytes until convalescence. Systemic administration of adenine arabinoside continues to demonstrate efficacy in treatment of herpes simplex encephalitis, and trials in herpes zoster are nearing their conclusion. Acycloguanosine, a topically administered antiviral compound, is effective in prophylaxis of genital herpesvirus infections in guinea pigs.